Wind turbines create power proportional to the swept area of their blades. Increasing the length of a wind turbine's blades increases the swept area, which produces more power. A wind turbine's generator, gears, bearings and support structure must be designed around the expected wind load and power production. At low wind speeds very long blades are desirable to get as much power as possible out of the available wind. At high wind speeds a wind turbine must control the power production and the mechanical loads developed. Eventually, the wind becomes high enough that the turbine must shut down to avoid damaging components, so short blades are desirable to keep the turbine producing power in high winds.
The choice of a rotor diameter for a wind turbine is a design trade-off between energy production in low winds and load limitation in high winds. Wind turbine manufacturers often sell a variety of rotor sizes for a given wind turbine model. These rotor sizes are optimized for sites that have a low, medium, or high annual average wind speed. However, the rotor size selected is always a compromise and there are conditions in which the turbine does not perform optimally because the rotor is too big or too small.
It would be desirable to provide a wind turbine with a large rotor that can produce a large amount of power in low wind conditions and a small rotor for limiting power and mechanical loads during high wind conditions. Such a turbine would preferably have a variable diameter rotor that can be adjusted to the current wind conditions.
Many variable diameter rotors have been designed for aircraft. One of the first is shown in U.S. Pat. No. 1,077,187 incorporated herein by reference. Many other variable diameter rotors, and improvements on them, have been patented since. Some of those patents, which are incorporated herein by reference are U.S. Pat. Nos. 3,768,923, 5,299,912, 5,636,969, 5,642,982, and 5,655,879. These rotor designs are all for use on aircraft of various sorts and they lack any teaching to utilize such a rotor on a wind turbine.
In the past when a turbine has been installed in a lower wind speed site than it was originally designed for, blades have been lengthened by adding hub extenders, which space blades out radially from their original mounting. Hub extenders accomplish the goal of increasing the swept area, but present the following disadvantages:                1. Hub extenders cannot be easily changed or removed, because they are relatively heavy devices. Since it requires a crane and hours of labor to change hub extenders on commercial sized wind turbines, they end up being left in place once they are installed.        2. Since hub extenders cannot be easily removed, extra loads are placed on the turbine every time high winds occur, and        3. The length of the hub extenders is limited by the strength of the existing drive train and other components. Either the turbine life is shortened, or the drive train, generator, and other components must be upgraded to withstand the higher loads caused by the longer blades. Since the entire drive train cannot be economically upgraded, the use of hub extenders is limited as a way of increasing the energy output of a wind turbine.        
It would be advantageous to provide a way of extending the length of wind turbine blades that is easily reversible so the wind turbine can take advantage of the extra power production of longer blades but not have the liability of long blades during periods of high wind.
Tip brakes are moveable blade sections located at the end of a turbine blade. One design shown in U.S. Pat. No. 4,715,782 incorporated herein by reference, reduces the efficiency of the blade by turning 90 degrees and causing drag. The tip portion of the blade is mounted to a shaft which allows the tip to be rotated 90 degrees to the blade. This acts as a drag which is used as a brake to slow the rotation of the blades. These devices allow the tip of the blade to move longitudinally a short distance in order to disconnect the tip from the end of the blade. Once free of the fixed portion of the blade, the tip is rotated the full 90 degrees to effectively destroy the capability of the blade to produce power. These tip brakes are not designed to operate at any position other than fully rotated, or fully lined up with the blade. Their function is to act as a safety device by reducing the ability of the turbine blade to make power. The longitudinal motion of the tip is minimal, and it serves the purpose of latching and unlatching the tip so it can be rotated.
Another design of tip brakes is shown in U.S. Pat. No. 4,710,101, which is incorporated herein by reference. This device uses a purely telescoping method of braking the wind turbine. A portion of the leading edge of the tip of the blade is extended exposing a non aerodynamic surface to the wind and exposing a non aerodynamic surface at the trailing edge of the blade tip. These factors combine to produce a braking effect. Even though the telescoping action in this design is greater than other tip brake designs the function remains the same; to inhibit power production of the wind turbine by altering the aerodynamic shape of the blade. While tip brakes are made up of a fixed and a moveable blade section, they:                1. do not allow the effective length of the blade to be changed;        2. do not improve the power output of the blade;        3. do not optimize the pitch of the end of the blade with changes in length, and        4. do not operate at positions intermediate to the fully deployed or fully lined up positions.        
It would be advantageous to provide a method of reducing the loads and power output of the wind turbine during high winds without inhibiting power production entirely, thus allowing for continued power production in high winds.